docs: def cast_to_seq(obj, alphabet=IUPAC.extended_protein):
      """Return a Seq representation of a string or SeqRecord object.
  
      Args:
          obj (str, Seq, SeqRecord): Sequence string or Biopython SeqRecord object
          alphabet: See Biopython SeqRecord docs
  
      Returns:
          Seq: Seq representation of the sequence
  
      """
  
      if isinstance(obj, Seq):
          return obj
      if isinstance(obj, SeqRecord):
          return obj.seq
      if isinstance(obj, str):
          obj = obj.upper()
          return Seq(obj, alphabet)
      else:
          raise ValueError('Must provide a string, Seq, or SeqRecord object.')